Expansion tank questions

[SteveW]

Hello,
My first post -- good site!

I have had problems with a vibrating sound in my house's plumbing, which I eventually realized was due to thermal expansion (I have a Watts PRV). The noise is relieved by opening the hot water tap for a second or two.

I bought a 5-gal expansion tank from Sears as I have 2 40-gal water heaters. In reading the instructions, they say that I need to make sure that I have a bypass- type PRV, which seems to me to negate the need for the expansion tank. Also, I'm supposed to permanently install a water pressure gauge and make sure PRV is set for a house pressure of 40 PSI.

Is this all liability prevention on the part of Sears, or do I need to do all of this? I have no idea what my current pressure is, but 40 PSI seems low to me, esp. with a sprinkler system. (Of course, I could reroute the sprinkler system to tap in before the PRV, but I am trying to do this job as simply as possible...)

Thanks!
Steve in Omaha, NE

[Plumber2000]

Sprinkler system should connect prior to the PRV.

If you have a PRV with a integrated bypass then expansion tank is not needed, if no bypass then an expansion tank is needed expansion is a problem, tank needs to be charged 5 to 10 lbs higher then working pressure on the house, test pressure with a hose adapter pressure gage.

[SteveW]

Thanks! I do wonder if the tank couldn't take a bit more pressure than the 40 PSI in the installation manual. I will put a boiler drain on the line so that I can hook up a pressure gauge and check out the pressure -- and I plan to put a ball valve on the expansion tank so that I can easily isolate it if/when it starts to leak (and this way I can check to see what happens to the house pressure with/without the tank).

Steve

[hj]

A bypass PRV does not function until the pressure in the house is above the "outside" pressure and then it keeps it at that point. But that pressure is why you installed the PRV in the first place. They must be trying to minimize liability because 40 psi is much too low unless that is all you can achieve, such as with a house well pump system.